Weather happens every day, but only some days have storms. Storms vary immensely depending on whether they're warm or cold, coming off the ocean or off a continent, occurring in summer or winter, and many other factors. The effects of storms also vary depending on whether they strike a populated area or a natural landscape. Hurricane Katrina is a good example, since the flooding after the storm severely damaged New Orleans, while a similar storm in an unpopulated area would have done little damage.

Figure 16.21: On a warm spring or summer day, air warmed near the ground rises and forms cumulus clouds. If warm air continues to rise, cumulonimbus clouds form.

Thunderstorms are extremely common. Across the globe, there are about 14 million per year; that's 40,000 per day! Most come and go quickly, dropping a lot of rain on a small area, but some are severe and highly damaging. Thunderstorms are most common when ground temperatures are high. This tends to be in the late afternoon or early evening in spring and summer. As temperatures increase, warm, moist air rises. These updrafts form first cumulus and then cumulonimbus clouds (Figure 16.21). At the top of the stratosphere, upper level winds blow the cloud top sideways to make the anvil shape that characterizes a cloud as a thunderhead (Figure 16.22).

Figure 16.22: Winds at the top of the stratosphere blow the top of a cumulonimbus cloud sideways to create the classic anvil-shape of a thunderhead.

Figure 16.23: A mature thunderstorm showing updrafts and downdrafts that reach the ground. This thunderstorm will no longer grow, since the base of the cloud is being cooled too much for convection to continue.

Clouds form when water vapor condenses. Remember that when water changes state from a gas to a liquid, it releases latent heat. Latent heat makes the air in the cloud warmer than the air outside the cloud and supplies the cloud with a lot of energy. Water droplets and ice travel through the cloud in updrafts. When these droplets get heavy enough, they fall. This starts a downdraft, and soon there is a convection cell within the cloud. The cloud grows into a cumulonimbus giant. Droplets traveling through the convection cell grow. Eventually, they become large enough to fall to the ground. At this time, the thunderstorm is mature, it produces gusty winds, lightning, heavy precipitation and hail (Figure 16.23).

Once downdrafts have begun, the thunderstorm can no longer continue growing. The downdrafts cool the air at the base of the cloud, so the air is no longer warm enough to rise. As a result, convection shuts down. Without convection, water vapor does not condense, no latent heat is released, and the thunderhead runs out of energy. A thunderstorm usually ends only 15 to 30 minutes after it began, but other thunderstorms may start in the same area.

Severe thunderstorms grow larger because the downdrafts are so intense, they flow to the ground. This sends warm air from the ground upward into the storm. The warm air feeds the convection cells in the cloud and gives them more energy. Rain and hail grow huge before gravity pulls them to Earth. Hail that is 1.9 cm (0.75 inch) in diameter is not uncommon. Severe thunderstorms can last for hours and can cause a lot of damage due to high winds, flooding, intense hail, and tornadoes.

Thunderstorms can form individually or in squall lines, which can run along a cold front for hundreds of kilometers. Individual storms within the line may reach an altitude of more than 15 kilometers (50,000 feet). In the United States, squall lines form in spring and early summer where the maritime tropical (mT) air mass from the Gulf of Mexico meets the continental polar (cP) air mass from Canada. In the United States, severe thunderstorms are most common in the Midwest.

Figure 16.25: Lightning strikes some places many times a year. Here, lightning is striking the Eiffel Tower in Paris.

Lightning is a huge release of electricity that forms in cumulonimbus clouds (Figure 16.24). As water droplets in the cloud freeze, positive ions line the colder outside of the drop. Negative ions collect in the warmer inside. If the outside of the drop freezes, the water inside often shatters the outside ice shell. The small, positively-charged ice fragment rises in the updraft. The heavier, negatively-charged water droplet falls in the downdraft. Soon the base of the cloud is mostly negatively-charged and the top is mostly positively-charged. The negative ions at the base of the cloud drive away negative ions on the ground beneath it, so the ground builds up a positive charge. Eventually the opposite charges will attempt to equalize, creating ground to cloud lightning. Only about 20% of lightning bolts strike the ground. Lightning can also discharge into another part of the same cloud or another cloud.

Figure 16.24: Lightning over Pentagon City in Arlington, Virginia.

Lightning heats the air so that it expands explosively. The loud clap is thunder. Light waves travel so rapidly that lightning is seen instantly. Sound waves travel much more slowly, about 330 m (1,000 feet) per second. If you were watching a lightning storm, the difference in the amount of time between seeing a lighting bolt and hearing its thunder clap in seconds times 1,000 gives the approximate distance in feet of the lightning strike. For example, if 5 seconds elapse between the lightning and the thunder, the lightning hit about 5,000 feet or about 1 mile (1,650 m) away.

Thunderstorms kill approximately 200 people in the United States and injure about 550 Americans per year, mostly from lightning strikes. Have you heard the common misconception that lightning doesn't strike the same place twice? In fact, lightning strikes the New York City's Empire State Building about 100 times per year (Figure 16.25).

Figure 16.26: The formation of this tornado outside Dimmit, Texas in 1995 was well studied.

Tornadoes, also called twisters, are the most fearsome products of severe thunderstorms Figure 16.26). Tornadoes are created as air in a thunderstorm rises, and the surrounding air races in to fill the gap, forming a funnel. A tornado is a funnel shaped, whirling column of air extending downward from a cumulonimbus cloud.

A tornado can last anywhere from a few seconds to several hours. The most important measure of the strength of a tornado is its wind speed. The average is about 177 kph (110 mph), but some can be much higher. The average tornado is 150 to 600 m across (500 to 2,000 feet) across and 300 m (1,000 feet) from cloud to ground. A tornado travels over the ground at about 45 km per hour (28 miles per hour) and travels about 25 km (16 miles) before losing energy and disappearing.

Tornadoes strike a small area compared to other violent storms, but they can destroy everything in their path. Tornadoes uproot trees, rip boards from buildings, and fling cars up into the sky. The most violent two percent of tornadoes last more than three hours. These monster storms have winds up to 480 kph (300 mph). They cut paths more than 150 km (95 miles) long and 1 km (one-half mile) wide (Figure 16.27).

Figure 16.27: This tornado struck Seymour, Texas in 1979.

Most injuries and deaths from tornadoes are caused by flying debris. In the United States, an average of 90 people are killed by tornadoes each year, according to data from the National Weather Service. The most violent two percent of tornadoes account for 70% of the deaths by tornadoes (Figure 16.28).

Figure 16.28: Tornado damage at Stoughton, Wisconsin in 2005.

Tornadoes form at the front of severe thunderstorms, so these two dangerous weather events commonly occur together. In the United States, tornadoes form along the front where the maritime tropical (mT) and continental polar (cP) air masses meet. In a typical year, the location of tornadoes moves along with the front, from the central Gulf States in February, to the southeastern Atlantic states in March and April, and on to the northern Plains and Great Lakes in May and June. Although there is an average of 770 tornadoes annually, the number of tornadoes each year varies greatly (Figure 16.29).

Figure 16.29: The frequency of F3, F4, and F5 tornadoes in the United States. The red region that starts in Texas and covers Oklahoma, Nebraska and South Dakota is called Tornado Alley because it is where most of the violent tornadoes occur.

Meteorologists can only predict tornado danger over a very wide region, a few hours in advance of the possible storm. Once a tornado is sighted on radar, its path is predicted and a warning is issued to people in that area. The exact path is unknown because tornado movement is not very predictable. The intensity of tornadoes is measured on the Fujita Scale (see Table 16.3), which assigns a value based on wind speed and damage.

A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a low pressure center. On the east side, winds come from the south and so are warmer than those on the west side. The swirling air rises and cools, creating clouds and precipitation. Cyclones can be the most intense storms on Earth. There are two types of cyclones: middle latitude cyclones and tropical cyclones. Mid-latitude cyclones are the main cause of winter storms in the middle latitudes. Tropical cyclones are also known as hurricanes.

An anticyclone, as you might expect, is the opposite of a cyclone. An anticyclone's winds rotate around a center of high pressure. Air from above sinks to the ground to fill the space left when the air moved away. High pressure centers generally have fair weather. Anticyclone winds move clockwise in the Northern Hemisphere, exactly the opposite of a cyclone. Since winds on the east side of the anticyclone come from the north and those on the west side come from the south, the east side tends to be colder than the west side of the high.

Middle latitude cyclones, sometimes called extratropical cyclones, form at the polar front when the temperature difference between two air masses is large. These air masses blow past each other in opposite directions. Winds are deflected by Coriolis Effect—to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This causes the winds to strike the polar front at an angle. Warm and cold fronts form next to each other. Most winter storms in the middle latitudes, including most of the United States and Europe, are caused by middle latitude cyclones (Figure 16.30).

Figure 16.30: A hypothetical mid-latitude cyclone affecting the United Kingdom. The arrows indicate the wind direction and its relative temperature; L symbolizes the low pressure area. Notice the warm, cold, and occluded fronts.

Figure 16.31: The 1993 "Storm of the Century" was a nor'easter that covered the entire eastern seaboard of the United States.

The warm air at the cold front rises and creates a low pressure cell. Winds rush into the low pressure and create a rising column of air. The air twists, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Since the rising air is moist, rain or snow falls.

Mid-latitude cyclones form in winter in the mid-latitudes and move eastward with the westerly winds. These two to five day storms can reach 1,000 to 2500 km (625 to 1,600 miles) in diameter and produce winds up to 125 km (75 miles) per hour. Like tropical cyclones, they can cause extensive beach erosion and flooding.

Mid-latitude cyclones are especially fierce in the mid-Atlantic and New England states where they are called nor'easters, because they come from the northeast. About 30 nor'easters strike the region each year. Most do little harm, but some are deadly. The typical weather pattern of a nor'easter is familiar to anyone who has lived in this region. First, heavy snow and ice cover the ground. Then, air temperature warms and rain falls. The rain hits the frozen ground and freezes, cloaking everything in ice (Figure 16.31).

Tropical cyclones have many names. They are called hurricanes in the North Atlantic and eastern Pacific oceans, typhoons in the western Pacific Ocean, tropical cyclones in the Indian Ocean, and willi-willis in the waters near Australia (Figure 16.32). By any name, they are the most damaging storms on Earth.

Figure 16.32: A cross-sectional view of a hurricane.

For a hurricane to form, sea surface temperature must be 28°C (82°F) or higher. Hurricanes arise in the tropical latitudes (between 10° and 25°N) in summer and autumn. The warm seas create a large humid air mass. The warm air rises and forms a low pressure cell, known as a tropical depression. Thunderstorms materialize around the tropical depression.

If the temperature within the cell reaches or exceeds 28°C (82°F) the air begins to rotate around the low pressure. The rotation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. As the air rises, water vapor condenses, releasing energy from latent heat. If winds frequently shift directions in the upper atmosphere, the storm cannot grow upward. If wind shear is low, the storm builds into a hurricane within two to three days.

Hurricanes are roughly 600 km (350 miles) across and 15 km (50,000 feet) high. Winds reach at least 118 km (74 miles) per hour. The exception is the relatively calm eye of the storm, which is about 13 to 16 km (8 to 10 miles) in diameter. The eye is calm because it is where air is rising upward.

Rainfall can be as high as 2.5 cm (1") per hour, resulting in about 20 billion metric tons of water released daily in a hurricane. The release of latent heat generates enormous amounts of energy, about 2,000 billion kilowatt hours per day. This amount of energy is nearly the total annual electrical power consumption of the United States. Hurricanes can also generate tornadoes.

Hurricanes are assigned to categories based on their wind speed. An estimate can be made as to the damage that will be caused based on the category of storm. The categories are listed on the Saffir-Simpson hurricane scale (Table 16.4).

Complete roof failure on small residences; major erosion of beach areas; major damage to lower floors of structures near shore

5 (devastating)

>251

>156

Complete roof failure on many residences and industrial buildings; some complete building failures

Hurricanes move with the prevailing winds. In the Northern Hemisphere, they originate in the trade winds and move to the west. When they reach the latitude of the westerlies, they switch direction and travel toward the north or northeast. Hurricanes typically travel from 5 to 40 kph (3 to 25 mph) and can cover 800 km (500 miles) in one day. Their speed and direction depend on the conditions that surround them. This uncertainty makes it hard for meteorologists to accurately predict where a hurricane will go and how strong it will be when it reaches land.

Damage from hurricanes tends to come from the high winds and rainfall, which can cause flooding. Near the coast, flooding is also caused by storm surge (Figure 16.33). Storm surge occurs as the storm’s low pressure center comes onto land, causing the sea level to rise unusually high. A storm surge is often made worse by the hurricane's high winds blowing seawater across the ocean onto the shoreline. Storm surge may rise as high as 7.0 to 7.5 m (20 to 25 feet) for up to 160 km (100 miles) along a coastline. If a storm surge is channeled into a narrow bay, it will greatly increase in height.

Figure 16.33: Storm surge effects on sea level.

Waves created by a hurricane's high winds and high tide further increase water levels during a storm surge. Flooding can be devastating, especially along low-lying coastlines like the Atlantic and Gulf Coasts. Hurricane Camille in 1969 had a 7.3 m (24 foot) storm surge that traveled 125 miles (200 km) inland.

Hurricanes can last from three hours to three weeks, but 5 to 10 days is typical. Once a hurricane travels over cooler water or onto land, its latent heat source is shut down and it will soon weaken. However, an intense, fast-moving storm can travel quite far inland before its demise. In September, 1938 a hurricane made it all the way to Montreal, Canada before breaking up. When a hurricane disintegrates, it is replaced with intense rains and tornadoes.

There are about 100 hurricanes around the world each year, plus many smaller tropical storms and tropical depressions. As people develop coastal regions, property damage from storms continues to rise. However, scientists are becoming better at predicting the paths of these storms and fatalities are decreasing. There is, however, one major exception to the previous statement: Hurricane Katrina.

The 2005 Atlantic hurricane season was the longest, costliest, and deadliest hurricane season so far. Although the hurricane season officially runs from June 1 to November 30, the 2005 hurricane season was active into January 2006. Total damage from all the storms together was estimated at more than $128 billion, with more than 2,280 deaths. Of the 28 named storms, 15 were hurricanes, including five Category 4 storms and four Category 5 storms on the Saffir-Simpson Scale.

Hurricane Katrina was both the most destructive hurricane and the most costly (Figure 16.34). The storm was a Category 1 hurricane as it passed across the southern tip of Florida. It was pushed westward by the trade winds, blowing over the Gulf of Mexico where temperatures were as high as 32°C (89°F). The warm Gulf waters and latent heat fueled the storm until it grew into a Category 5. As it moved through the Gulf, the mayor of the historic city of New Orleans ordered a mandatory evacuation of the city. Not everyone was willing or able to comply.

Figure 16.34: Hurricane Katrina nears its peak strength as it travels across the Gulf of Mexico.

Figure 16.35: Flooding in New Orleans after Hurricane Katrina caused the levees to break and water to pour through.

When Hurricane Katrina reached the Gulf Coast, it had weakened to a Category 4 storm. Even so, it was the third strongest hurricane to ever hit the United States. The eye of the storm struck a bit east of New Orleans, buffeting the area around Biloxi, Mississippi with the worst direct damage. The initial reports were that New Orleans had been spared. But as water began to rise in the lowest lying portions of the city, officials realized that the storm surge had caused the levee system to breach. Eventually 80% of the city was underwater (Figure 16.35). By the end of that horrible period, around 2,500 people were dead or missing from the Gulf Coast, most of them from New Orleans. Over two hundred thousand of people left New Orleans as a result of the hurricane, and many have not returned due to loss of their homes and livelihood.

Snow so heavy that visibility is 2/5 km (1/4 mile) or less for at least three hours; near zero visibility for a severe blizzard.

Figure 16.36: A near white out in a blizzard in Minnesota.

Blizzards happen across the middle latitudes and toward the poles. They usually develop on the northwest side of a mid-latitude cyclone. Blizzards are most common in winter, when the jet stream has traveled south and a cold, northern air mass comes into contact with a warmer, semitropical air mass. The very strong winds develop because of the pressure gradient between the low pressure storm and the higher pressure west of the storm. Snow produced by the storm gets caught in the winds and blows nearly horizontally. Blizzards can also produce sleet or freezing rain.

The snowiest, metropolitan areas in the United States are Buffalo and Rochester, New York. These cities are prone to getting lake-effect snow. While other locations can have lake effect snow, the greatest amount is on the leeward side of the Great Lakes. In winter, a continental polar air mass travels down from Canada. As the frigid air travels across one of the Great Lakes, it warms and absorbs moisture. When the air mass reaches the leeward side of the lake, it is very unstable and it drops tremendous amounts of snow. Buffalo is on the leeward side of Lake Erie and Rochester is on the leeward side of Lake Ontario.

While lake effect snow is not a blizzard, the two can work together to create even greater snows. The Great Lakes Blizzard of 1977 was created mostly by the passage of a cold front over the area. The snowfall was aided by lake effect snow coming off of Lake Ontario, which had not yet frozen that winter.

Although not technically storms, extreme heat and drought are important weather phenomena. A heat wave is defined as extreme heat that lasts longer than normal for an area. During a heat wave, a high pressure zone sits over an area and hot air at the ground is trapped. A heat wave can occur because the position of the jet stream makes the area hotter than it is normally. For example, if the jet stream is further north than usual, hot weather can also be found north of where it is usual. Winds coming from a different direction can also make a region hotter than normal. Temperatures that would not ordinarily be too hot may create a heat wave if the humidity rises too high.

More people die from extreme heat on average each year than in any type of storm. The Chicago Heat Wave of 1995 killed about 600 people who did not have access to air conditioning. The world was shocked in July and August 2003, when between 20,000 and 35,000 died in a European heat wave, mostly in France (Figure 16.37).

Figure 16.37: Temperature anomalies (outside of the normal, expected range) across Europe in the summer of 2003. France was the hardest hit nation.

Drought strikes a region if it has less rainfall than normal for days, weeks, or years, depending on its location. A normally wet city enters drought at a much greater rainfall level than a city located in the desert. A location may also be experiencing drought, even if it receives rain, if the rain falls so that it is useless to humans. For example, a heavy rain may run off a dried out landscape rather than sinking into the soil and nourishing the plants.